Did they know that you might be selling the house one day?
Why don't they choose to buy said house that they have been having a sweet deal on? for how long? If they have only been paying utilities, (no mortgage? no taxes?), then there should have been plenty of time to save for a down payment on their own home.
Have you thought of asking them if they wish to purchase this home, since they are already living in it? If this is an idea you might have, I would first get it it appraised, (not telling them why, just stating tht it needs to be done to possibly sell home), and then go to them with an offer of how much you are willing to accept?
I would not email, facebook, etc. unless Rien will not talk to you by phone. I would wait a week or so, and think of a calm reply.
Somethng along the lines of, "Rien, there seems to have been some miscommunication. I would like to clear this up if possible. First off, start with what was actually said to her other half about the possible selling of the house and state what is true. Then, I would call her out on her FB status and let her know that you were really hurt by this. You understand she might have have been upset by the miscommunication, but she should have called you first and discussed it before saying those things to everyone about you that can now never be taken back. And you think it is really poor taste, especailly when you offered your home to them with only paying utilities. Where else would she have found this kind of deal? You hope that you can get together to talk and discuss this situation to eliminate any further communications."